Channel info

John Orlando john at
Sun Jan 30 17:34:01 UTC 2011

On Sun, Jan 30, 2011 at 11:20 AM, Bogdan Alecu <b.alecu at> wrote:

> Hi Mad,
> Thanks for explaining me this. The case of the TMSI is different from
> network to network: some operators change it for every x minutes, others
> after a powercycle (at least here). You say that gsm tries to avoid sending
> out IMSIs but still some are sent. If I would send an SMS to the phone - so
> it will get a paging request - I should see the IMSI also in that list?

If you send an SMS to the phone, the "typical scenario" would be:
   -Network pages the mobile, most likely with its TMSI (as stated before)
   -Mobile will send a RACH to request a dedicated channel
   -Network will assign the mobile to a dedicated channel (typically a
   -Network will perform contention resolution on the dedicated channel to
ensure that it is the correct mobile (to cover the rare case where two
mobiles RACH at the exact same time)
   -Network may or may not begin ciphering at this point
   -Network will deliver the SMS payload to the mobile through a series of
messages on the dedicated channel
   -Mobile will ack each message, letting the network know that it has
received what was sent
   -Network will release the dedicated channel, the mobile will go back to
its idle mode (monitoring the paging channels etc), and the mobile will then
have the SMS

It is up to the network to decide what ID type it wants to use to page a
mobile, and this is dependent on a number of factors.  Almost always it uses
the TMSI, sometimes IMSI, and _very_ rarely an IMEI.  It is certainly
possible that the network can also request identity info (IMSI, IMEI) from
the mobile on the dedicated channel, as well as a whole host of other
message requests while on the dedicated channel.

So in short, unless you're lucky enough to have the network page you with
your IMSI, it is unlikely you will see it.

John Orlando
CEO/System Architect
Epiq Solutions
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